It is known that textile printing machines generally comprise an endless cloth or belt surrounding two cylinders one of which is rotated and above which belt are arranged successive printing cylinders or flat screens adapted to print patterns of different colors onto a fabric web carried along by the belt and sandwiched between this belt and the printing cylinders.
The endless cloth or belt generally has a sandwich-like structure with several more or less thick layers of elastomeric material, of fabric etc adapted to permit the printing of the fabric resting upon the belt through compression of the printing cylinders or flat screens upon the fabric.
The endless belt is of course driven with a constant speed by the travelling speed of the belt however remains irregular in spite of all in view of the possible local defects or variations in the inner structure of this belt, these structural defects or variations imparting a somewhat variable resiliency to the belt.
It results therefrom a "shift" or "registering inaccuracy" of the colored patterns printed in overlying relationship onto the fabric disposed upon the endless cloth or belt by the successive printing cylinders or flat screens.
Such a registering inaccuracy may reach a few tenths of a millimiter which is detrimental to the appearance of the fabric. A registering inaccuracy which is greater than about one tenth of a millimeter may indeed not be tolerated.
Therefore it is essential to very accurately control the local travelling speed of the endless cloth or belt in order that the registering inaccuracy of the fabric does not exceed the value referred to hereinabove since otherwise the said belt could not be suited to a fabric printing machine and should be discarded.
Heretofore to control the accuracy of the printings people were satisfied to watch with a magnifying glass for instance directly on the printing machine whether there was a registering inaccuracy between two patterns printed onto the fabric.
As it will be appreciated such a method is empirical, not very reliable, of time-consuming performance and expensive in labour.
It is therefore appropriate to propose a method of and a machine for continuously controlling an endless belt which would permit to ensure a registering inaccuracy equal to zero or reduced to an admissible minimum when the said belt is fitting a fabric printing machine.
The present invention achieves that goal.